The present invention relates to the catalytic production of lactide and more particularly to its direct production from lactic acid.
For purposes of this application, the following definitions apply:
L.sub.1 A: lactic acid or 2-hydroxypropionic acid PA1 LD: lactide or 3,6-dimethyl-1,4-dioxane-2,5-dione PA1 L.sub.2 A: lactoyllactic acid or lactic acid dimer PA1 L.sub.3 A: lactoyllactoyllactic acid or lactic acid trimer PA1 L.sub.n A: n-oligomer of lactic acid.
The DP or degree of polymerization of lactic acid is "n".
Lactic acid has one asymmetric carbon atom and, thus, can be found in two enantiomeric forms. Lactide, on the other hand, has two asymmetric carbon atoms so that it can be found in three steroisomeric forms: L-lactide in which both asymmetric carbon atoms possess the L (or S) configuration; D-lactide in which both asymmetric carbon atoms possess the D (or R) configuration; and meso-lactide in which one asymmetric atom has the L configuration and the other has the D configuration. L-lactide and D-lactide are enantiomers while D,L-lactide is the meso species. In the production of lactide from lactic acid, it would be advantageous if the absolute configuration of the lactic acid feed was maintained in its conversion to lactide. Enantiomeric lactide, especially L-lactide, has utility in the production of polymers, especially in the production of environmentally degradable polymers such as proposed in commonly-assigned U.S. applications Ser. Nos. 387,670; 387,676; 387,678; and 386,844.
Heretofore, production of lactide from lactic acid has proceeded by the initial formation of oligomeric lactic acid, L.sub.n A, such as by dehydration of aqueous lactic acid, followed by a catalytic transesterification reaction known as "back-biting" as illustrated below: ##STR1##
As illustrated above, back-biting depolymerization of L.sub.n A results in the production of lactide. Catalysts proposed for such a reaction include tin powder, tin halides, or tin carboxylates (EP Publication 261,572); tin alkoxides (U.K. Pat. No. 1,007,347); and zinc or tin (EP Publication 264,926).
Direct conversion of lactic acid into lactide with or without preservation of absolute configuration of asymmetric atoms is not shown in the art.